This study deals with the development of an innovative weigh-in-motion (WIM) sensor. An electrically conductive nanocomposite material based on a mixture of graphene supported on sepiolite and carbon nanotubes was developed. Deposited on bituminous mix with copper electrodes, it is used as a force sensor. We detail the sensor fabrication process and study its sensitivity to a compressive force.

We present a novel integration-driven approach to the design of multi-scale multi-physics sensors and systems. We implement this method to model, design, fabricate and characterize a thin, conformable low-cost impact detection sensor based on assemblies of piezoelectric GaN nanowires. When suitably assembled, the latter demonstrate a macroscale additivity of their nanoscale intrinsic properties, which enables to appeal to classical fabrication techniques and exploitable electronic readouts at the system level. We also exploit multi-level simulations to provide useful insights of adapted application-driven integration solutions for these new forms of sensors. We demonstrate the potential of such application-targeted, fully-integrated and modular systems to accommodate to the stringent requirements of structural health monitoring (SHM).

In this work there have been investigated the potential usage of the CNT's as strain sensors for the structural health monitoring based on the spray coatings. Experimental work was performed on the metal and glass-reinforced composites. Multiwalled Carbon Nanotubes (MWCNTs) were mixed with different matrix materials (acrylic and epoxy) and then applied to the test material with the use of two techniques (screen printing and spray coating). Futhermore, sensors were investigated using SEM. Response of the sensors was measured due to the tensile test of the specimens..

This work proposes a new type of low-cost strain sensor, based on piezoresistive carbon nanotube (CNT) network deposited on a flexible substrate. Experimental results show that the strain can be reliably measured thanks to the highly linear piezoresistive behaviour of the CNT network and thanks to temperature compensation capabilities. Moreover, the experimental results show the capability of measuring multiple loading cycles. The performance and the range of sensitivity of the device, suggest possible usage in the domain of embedded monitoring, in particular the detection of micro-strain and micro-cracking in concrete. In order to target this domain, a wireless RFID solution to embed the sensor into concrete is provided.

The Acoustic Emission (AE) technique has been shown as capable in detecting and locating fatigue crack damage in metallic structures. However there are significantly fewer studies investigating its potential for fatigue crack length estimation. Information on the extent of crack growth would enable prediction of the remaining useful life of a component using well established fracture mechanics principles. This would improve the prospects of AE for use in structural health monitoring applications where detection and monitoring of crack lengths is required. A new approach for deducing absolute crack length has been developed based on correlations between AE signals generated during fatigue crack growth and corresponding cyclic loads. An empirical model to generate crack length was derived using AE data generated during fatigue crack growth tests in 2 mm thick SEN aluminium 2014 T6 specimens subject to a tensile stress range of 52 MPa and an R ratio of 0.1. The model was validated using AE data generated in separate tests performed with a stress range of 27 MPa. The results showed that predictions of crack lengths over a range of 10 mm to 80 mm can be obtained with the mean of the normalised absolute errors ranging between 0.28 and 0.4.

Despite extensive developments in the field of Acoustic Emission (AE) for monitoring fatigue cracks in steel structures, the implementation of AE systems for large-scale bridges is hindered by limitations associated with instrumentation costs and signal processing complexities. This paper sheds light on some of the most important challenges in the utilization of AE systems for steel bridge decks. These challenges are mainly related to the multi-modal character of guided waves, and the expensive installation of AE instrumentation. A quasi-beamforming solution which alleviates the above-mentioned challenges was introduced and successfully evaluated on a real-scale bridge segment in a laboratory environment. The solution was also implemented on a real bridge structure, demonstrating the benefit of the proposed configuration where the transducers are closely spaced for reduced installation costs.

Diagnostic and monitoring approaches, able to detect and classify the damage and wear process, are becoming of increasing importance. Especially for friction wear-related phenomena which are difficult to measure directly during the operation and their diagnostic has been usually restrained to offline examinations. Permanent contact and repetitive sliding motions between two surfaces lead to material changes. Due to different wear mechanisms, sudden structural changes appear, emitting energy in form of elastic waves known as Acoustic Emission (AE). Therefore, a correlation between the emitted AE and damage level can give important information about the process state and the related knowledge can be used for automated supervision. This contribution introduces an advanced method for wear states identification and classification by means of AE technique and fuzzy-based multi-class classification approach. Compared to the previous publications, here the sequential effect of the motion trajectory is investigated. To establish a relationship between wear mechanism and AE signals, frequency-based feature selection using Continuous Wavelet Transform (CWT) was performed. Five wear process stages were detected during experiments. Results show that the behavior of individual frequency components changes when the wear-related effect changes. Using the CWT transformed signals, statespecific pattern are generated to classify signal features related to specific states. Results show that the introduced method can be used as an online monitoring method for material detection and characterization.

Acoustic emission technology was applied to assess the damage in the wind turbine blade. It was tried to apply a new source location method, which has a developed algorithm ourself with energy contour mapping concept. Firstly, we acquired energy based contour map database for tested blade section. And then, we measured the activities and the intensity of each arrival signals for several types of damage sources. That is, this study aims to locate and evaluate the damages such as internal damages or foreign impacts etc.. In this study, we focused to enhance a source location method with energy contour map which is developed already, and to develop a new damage index for more clear damage identification. For damage indexing, we found the correlation between corresponding energy and distance from source. Then, after calculating the location of damage source, we can doing more quantitative assessment using pre-acquired damage indexing. Consequently, the applicability of new source location method was confirmed by comparison of the result of source location and experimental damage location. From several experimental results, new suggested method of damage index identification showed very good performance for assessment of damages in the hybrid composites structures.

This paper is dealing with the development of a structural health monitoring (SHM) system implemented on a composite footbridge during the regional project ÒPays de la Loireî called DECID2. The SHM system was made out of complementary techniques: strain sensors based on optical fibers (out of concern in this present work) and ultrasonic techniques that are presented in this document. Due to the huge size of the composite bridge (20 m * 3 m), only its most critical areas are monitored as the assembling parts and the most solicited areas. To access the structural integrity of the footbridge, two complementary monitoring strategies were presented in this paper: a real-time acoustic emission monitoring system to detect fibre breaks and a monitoring system using guided waves to evaluate the resin degradation. Both of these SHM systems use the same miniature ultrasonic patches that are used alternately as acoustic emission sensors and as ultrasonic guided waves actuators. The first step of this work was to develop these patches, and then to set up each monitoring systems and characterize their damage sensitivity. Finally, two composite footbridges were built at the EMC2 Technocampus and IFSTTAR in France to serve as demonstrators.

Topological imaging is an emerging ultrasonic imaging method based on two computations performed for the so-called reference medium. The reference medium should correspond as close as possible to the experimental medium in the absence of defects. The topological image will then highlight all the differences between the investigated experimental medium and the reference medium. The presented work aims at applying this method to the detection, location and imaging of defects in composite plates. If the propagation can be properly simulated and the associated experience performed, the defects of the medium can be imaged whatever the complexity of the propagation process. Wave propagation in composite plates is here computed considering an equivalent homogeneous anisotropic medium in the frequency regime. First a numerical experiment is presented. It consists in the investigation of a strongly anisotropic medium using two different wave modes simultaneously. Three defects are accurately located and imaged in the medium. Secondly, a real experiment is investigating an impacted quadratic composite plate is investigated with a single guided mode. Two impacts are well detected and located.

Concrete is a widely used construction material by virtue of its cost and mechanical properties. Due to its low tensile strength however, concrete is very sensitive to crack formation. Cracks in concrete are responsible for significant inspection, maintenance and repair costs. In order to optimize structural health management, Non-Destructive Testing (NDT) has been extensively studied. Among all NDT techniques, ultrasonic methods are considered advantageous by providing information on mechanical properties in areas not directly accessible from the surface. Recent studies have led to developing nonlinear ultrasonic methods to increase the sensitivity to damage making possible the detection of large cracks/notches and the monitoring of crack evolution. However, the detection of small cracks in concrete remains a great challenge for NDT techniques. In this study, an ultrasonic method, based on nonlinear acoustic mixing of coda waves by lower-frequency swept pump waves, providing for an efficient global detection of small cracks in concrete is presented. By simultaneous comparison, for uncracked and cracked mortars, of ultrasonic velocity variations and decorrelation coefficient between the unperturbed and the perturbed signals for different pump amplitude, the method allows to accurately detect very small cracks with widths of around 20 mm correlated with velocity variations of approximately 0.01%. This method is reproducible and able to provide a simple means for differentiating damaged and sound concrete. Attention must be paid however to the material evolution during the time span of both a single experiment and the entire experimental campaign as a consequence of the presumed high sensitivity of the observables. Several applications of this technique could be developed in the field of civil engineering, although the power of the pump source would constitute a limitation. For example, the detection of small cracks causing leakage could be performed without any need for percolating fluid.

Coda Wave Interferometry (CWI), a method to evaluate subtle changes of elastic wave velocity in a medium, has been proven to be effective to detect small changes or ultrasonic velocity in concrete caused by load, temperature, moisture, damage or other means. While classical CWI is just able to determine velocity changes globally in relatively large areas between and around pairs of transmitters and receivers, several approaches have been proposed to identify the area affected by the changes more precisely. Most of them are based on the calculation of sensitivity kernels for de-correlation of signals measured at a specific state against a reference. Others follow simplified approaches. In a laboratory setup a concrete specimen of 1.5 by 1.5 by 0.5 m m³ was compressed at a certain point by a thread rod, screw nuts and 10 by 10 cm m³ load distribution plates including a load cell to measure the effective load. Maximum loads of 20 to 100 kN (more than one orders of magnitude below the compressive strength of the concrete) have been applied in 5 to 10 kN steps in various cycles. The specimen is equipped with 18 embedded ultrasonic broadband piezo transceivers (60 kHz central frequency). Ten of these receivers have been connected to a multiplexer and ultrasonic transmitting and receiving equipment in a way that allowed almost continuous two way measurements between all sensor pairs. Some of the sensors have been just a few cm away from the center of the load, some almost 1 m. Even simple ways to evaluate the data (e.g. crosscorrelation between signals at different load states) allowed pinpointing the load center at least approximately. A more detailed data evaluation either using CWI or even more one of the more sophisticated localization algorithms gave Òsharperî results in terms of localization and a better correlation between load and velocity change/de-correlation. Referencing the results to the transmitter-receiver pair least affected by load change has led to further improvement. The results are used in upcoming monitoring systems for concrete structures.

Traditional ultrasonic imaging techniques encounter difficulty on complexes material such as concrete, which is in part due the use of coherent waves in a very heterogeneous material. From this angle, technique called LOCADIFF has been developed for monitoring heterogeneous media using multiply scattered waves [1, 2]. We consider that modifications in the medium are equivalent to the presence of extra scatterers, which are characterized by their effective scattering cross-section σ,. Within this view, LOCADIFF allows to locate the modification by measuring the spatio-temporal de-correlation of multiply scattered waves and by solving the corresponding inverse problem. Based on LOCADIFF, a newly developed imaging technique has been reported [3]. By mapping the intensity of modification on localized microstructure, the new technique is able to detect perturbations at multiple locations. Here we present the application of this new technique on a real-size 15 tons concrete structure for imaging early-stage cracking procedure issued from four point bending load, as part of the CEOS.fr project. Experimental results show that this technique can not only locate cracks that appeared simultaneously at multiple locations, but also detect them and observe their developments since an early-stage.

Utilization of coherent phase information in complex wave fields formes the basis of interferometric time series analysis. The concept is known since decades, but until about 15 years ago there have been no practical implementations. Meanwhile seismic interferometry is used in a wide range from investigations of the earthÕs deep crust to engineering applications. Focused on monitoring and imaging the MIIC project as part of the German GeoTechnologien program has contributed to this development. Special attention was given to the transfer of methodology to different length scales that range from centimeters, in laboratory applications, over geotechnical scales to even kilometers in seismological applications. General purpose methods and open source software was developed, which can be used on all scales. The core of the MIIC software is a Python library organized in different modules for various processing tasks. A graphical user interface facilitates the creation of processing routines by visualizing connections and dependencies of variables and by checking the consistency of data types. Example applications have included carbon sequestration, salt mine and railrod embankment monitoring as well as imaging changes in concrete constructions.

A high precision can be achieved with ultrasonic coda waves to monitor the mechanical properties of concrete material (~10-5 in relative). This high sensitivity can be used to detect damage initiation and to closely follow concrete mechanical properties evolution with time. This advantage is counterbalance by the influence of environmental conditions making reproducibility of any experiment in concrete a challenging issue especially when in situ measurements are performed. Indeed thermal and water gradients present in the thickness of the structures (several decimetres) cannot be controlled and must be compensated. In this paper a protocol to remove environmental bias is proposed. Furthermore, to follow the apparition of a tensile crack in a metric size structure, non-linear mixing of coda wave via frequency-swept pump waves is tested. It is shown that, when the crack is closed (by pre-stressing cables), it is still possible to detect its presence. The non-linearity of the cracked zone remains at a high level, comparable to the case when the crack was open.

Scanning laser systems are a class of ultrasonic guided wave systems that have shown great promise for the inspection of complex structures because of the high spatial resolution of their resulting data. In this study, the focus is on applying full-field signal processing techniques to automatically detect and characterize fastener damage in aircraft components with realistic structural complexity. The particular testbed being considered is a composite tail section with several rivet lines. Artificial damage was introduced into several of the rivets with different magnitudes and orientations. While trained users of the laser scanning system can often detect the damage modes by simply viewing the resulting images, a more automated and quantifiable approach is desired to increase the consistency and quantify the confidence of the inspection system. In order to characterize the state of each rivet, a polar wavenumber processing technique is proposed. First, the full-field data is transformed to a new polar coordinate system centered on a particular rivet. A 3D Fourier transform then transforms the data to the frequency-wavenumber domain. Finally, the data is windowed to isolate the waves propagating in the positive radial direction. Analyzing the scattering pattern from each rivet in turn provides information on the health of each rivet. Damage detection may be accomplished either by comparison of the extracted scattering pattern with a model of the expected scattering pattern, or by an absolute reference-free method comparing rivets to one another to identify outliers. The results show that there is the potential to provide quantified damage detection and characterization for fastener damage using laser-excitation guided wave inspection systems.

Remote engineering systems are valuable tools to give visual assistance and remote support e.g. in NDT (Non-destructive Testing) or SHM (Structural Health Monitoring). They allow discussing a second opinion with a remote expert and thus reducing the human factor during testing and monitoring. For an optimal impression of the situation, the second person requires both a camera view of the location and the screen view of the system used. The OMA system (Online Maintenance Assistance) implements this two-view collaboration. Remote partners can see and actively control the equipment, while observing details of the location in the camera window. Due to varying working conditions, screen signals and communication properties, an adaptive compression for both signals (camera and screen) is proposed. This permits to always maintain the best possible visual quality for the assessment performed by the remote partner. The OMA screen compression is valuable for dynamic signals like in most NDT applications. Slower NDT and SHM applications benefit from a smoother and more realistic handling of the controlled software. Currently compression profiles are switched manually, but a classificator will soon allow automatic adaptation. Since most of the OMA system is browser based, it can be used on laptops or tablets and will be available for the iOS based iPad soon. The compression benefits can be used for sensor data compression and direct sensor data communication as well. New OMA integrations include other testing technologies, devices and other and other signal related aspects. The authors are interested in other collaboration scenario with the neeed for multiple views on cameras and screens.

At laboratory level, secondary bonding of Acousto-Ultrasonic sensor systems is well established and has proven its reliability in applications from coupon level up to flight test installations. However, the applied sensor secondary bonding is a manual process with high amount of required auxiliaries and tools and is hence associated with high costs. In transition from sensor installation under laboratory conditions to the installation of large-scale sensor networks within an aircraft serial production process, costs associated with sensor installation become a decisive factor. By developing the sensor co-bonding approach, an installation method for fiber reinforced plastics host structures is created which requires significantly less work, auxiliaries and tools. Yet, while a considerable reduction of installation cost seems achievable, the application of sensor co-bonding as an industrialized process for aircraft primary structure is clearly more challenging. The present paper outlines specific advantages of sensor co-bonding and discusses challenges ahead on the way towards an industrialized sensor installation on aircraft primary structures.

This investigation is studying the behaviour of a bespoke wireless impact locator design for monitoring structures against any kinds of impacts. The device performance was evaluated against its location repeatability, its ability to last the required service life, its power consumption and its wireless range. The main variables in this investigation were the sensor sensitivity, the wireless node data repeatability and the power supply output voltage. The investigation identified which power source to be used, what is the strategy to determine the remaining power life, as well as the feasibility to use such technology for impact location.